Reason for Carburetor Malfunction | Starter Plunger May be the Cause of the Slower System’s Density!

There are two types of mechanisms used to start the engine when cold on a carburetor specification motorcycle: the choke and the bi-starter. The purpose of both is to thicken the mixture so that it is easier to burn, but they do exactly the opposite. If the engine is warmed up but the mixture is still thick and covered, check the condition of the starter plunger if the motorcycle is fitted with a bi-starter.

The difference between shutting off the air and pumping a lot of gasoline.

Mikuni carburetor for the Kawasaki KZ1000J. No problem at mid to high speed, but a little bumpy at idle and low rpm. Replacing the stock carburetor on an out-of-print vehicle with a modern racing carburetor such as the TMR or FCR often results in a more stable idle, but this is often due to the age of the parts. The role of the carburetor is to shut off the air and gasoline from the engine that wants to breathe as much air as possible, and the wear caused by the negative pressure of the intake is more pronounced in the area where the throttle opening is small and the negative pressure is large.

When starting the engine in the winter when the temperature is low, carbureted motorcycle owners always use the choke mechanism for starting. Some riders may understand that by pulling the lever on the left switch of the steering wheel or the knob on the carburetor body, it makes it easier to start the engine. If you are an owner of a fuel-injected model, you may think that you don't use the choke regardless of the season.

Parts such as slow jets and main jets are set on the assumption that the engine is warmed up. However, in the winter when the temperature is low, it is difficult for the gasoline to atomize, making it difficult to start the engine after it has warmed up. This is why we have incorporated a special starting choke mechanism that can create a much thicker mixture than normal.

Incidentally, fuel-injected vehicles do not have a manually operated choke mechanism, but an outside temperature sensor built into the ECU, which directs fuel injection, automatically applies a correction appropriate for the ambient temperature. This correction, known as cold weather correction, improves the starting performance of cold engines by injecting more gasoline than in normal operation.

In a nutshell, there are two ways to thicken the mixture with the carburetor choke mechanism. One is a "choke" and the other is a "bi-starterIt is a choke. As the word choke means to squeeze or choke, the carburetorPut a lid on the passage and physically close it.It is made to work by When the engine is started, if you force the carburetor to shut off the air it is trying to suck in, it will try to pull gasoline out of the float chamber instead of air. This makes the mixture thicker and improves cold starting performance.

In contrast to this, apart from the venturi through which the air and mixture pass when driving. Dedicated passages for starting to create a thicker mixture.is a bi-starter. In a bi-starter system, the bi-starter plunger (plunger) opens a passage when the lever or knob is operated. Gasoline flows through this passage and exits on the engine side rather than the throttle butterfly. Therefore, when the throttle is closed and the by-starter is operated to start the engine with the plunger open, a large negative pressure is applied to the by-starter path and gasoline is sucked out, creating a thick, flammable mixture.

A choke that squeezes air and a bi-starter that pumps a lot of gasoline have the same goal of creating a thicker mixture, but the specific means are the exact opposite. This is the same relationship as the pilot screw and air screw. This is the same relationship between the pilot screw and the air screw that is the key to carburetor throttle adjustment. The pilot screw adjusts the volume of the mixture, so when it opens, the mixture increases and becomes thicker, and when it closes, it becomes thinner. The air screw adjusts the amount of air used to make the mixture, so when it opens the air increases and the mixture becomes thinner, and when it closes it becomes thicker.

Gasoline leakage is caused by the plunger opening a little

The plunger, the main component of the by-starter, is built orthogonal to the venturi. On this carburetor, the round shaft opens and closes the venturi. This is with the choke knob pushed in and the plunger blocking the starter path.

Pulling the choke knob on the clutch end pulls the plunger out. At this time, the starter path opens inside the carburetor and a thick mixture is sucked out toward the combustion chamber. Since the choke closes the venturi, the engine will not work if the air is shut off, but since the by-starter is a separate system from the venturi, it is not impossible to ride even if gasoline is supplied from the starter path, so it is sometimes difficult to grasp the state of gradual thickening.

The lever that the shaft goes through is hooked onto the end of the plunger to open and close it. The spring between the lever and the shaft applies a force in the closing direction to the plunger, but depending on the friction of the plunger, it may not be able to push through. There are some examples such as Keihin's CVK carburetor where the spring is built into the plunger shaft itself.

The plunger of the bistarter is a metal cylinder with a rod at the end of it and a rubber plate set into the end of the cylinder. When the plunger, which strokes slightly, is closed, the rod and rubber plate close the bistarter passage and shut off the gasoline.

However, if the bistarter is left on for some reason, the slow mixture will become thicker, causing the engine to bog down at low throttle and low engine speeds, or cause the plugs to rattle. If you have a choke that closes the venturi forcibly, the engine speed will not go up even if you open the throttle because no air can get in, but if you have a bi-starter, air can pass through the venturi, so the slow mixture will be bumpy but the top will turn, so you can ride even if the plunger is not completely closed. In the case of the bi-starter, the throw is bumpy but the top turns.

The plunger incorporates a small spring that works in the closing direction, but if a foreign object becomes lodged in the gap between the plunger and the carburetor body and movement becomes poor, the spring tension alone may not be enough to return the plunger. In the case of a negative pressure carburetor, air for the by-starter may be taken in from the negative pressure chamber at the top of the carburetor. When the throttle is closed and the by-starter is open, there is a strong negative pressure in this path, so if the stock air cleaner box is removed for customization, dust and debris can be sucked in from here.

If the plunger is opened and closed while the inside of the carburetor is dirty, it is no wonder that the movement of the plunger damaged by dust and dirt is poor. All the parts inside the carburetor are assembled with precise clearances, and the plunger also operates with a perfect sense of resistance. The plunger also operates with a sense of exquisite resistance, so the slightest foreign object can be fatal. The condition of the rubber at the end of the cylinder is also important. As long as the rubber blocks the passage when the plunger is closed, there is no leakage of gasoline, but if its flexibility deteriorates over time, gasoline may be gradually sucked out.

Many people may think that because the bi-starter only works when the motorcycle is started, it doesn't matter when riding a motorcycle. But when you stop at a traffic light as soon as the throttle opening becomes zero and the intake negative pressure reaches the maximum, the carburetor tries to suck in as much air as possible from where there is a possibility that air can be sucked in. If the plunger can't close due to friction, or if the rubber deteriorates and can't seal completely, it's possible to get a thick mixture from the bi-starter system for starting. If the plunger can't close due to friction, or if the rubber deteriorates and can't seal completely, it's possible that the by-starter system will suck in a thick mixture for starting and cover it.

Pay attention to the position of the plunger lever for the 4-stroke carburetor.

If you remove the top cover of the carburetor and the negative pressure piston, you can see part of the plunger at the bottom of the negative pressure chamber on the cylinder head side from the throttle valve. When the choke is pulled, the plunger slides to the right side of the image and the starter system opens, and the air in the negative pressure chamber is sucked in and mixed with the gasoline to form a thick mixture.

When I removed the plunger that I felt stuck when operating the choke, I found that it was matted with fine scratches in both circumferential and stroke directions. The scratches looked like they were caused in the starter path rather than by a foreign substance from outside. It is important to note that sandblasting the carburetor may leave some media in this area.

After cleaning the body side with parts cleaner, apply metal polish to the plunger and slide it in place. If the starter path closes or doesn't close depending on the position of the plunger, it could cause the carburetor to perform better or worse. The rubber at the tip was flexible, so it appears that friction from the scratched plunger is the cause of the poor performance at low openings.

If you polish too hard, it will lead to wear, so you need to rotate it gently in the axial direction to get a good hit. The friction was large in the dry state, but after reapplying the compound several times and rubbing it together for a while, it started to rotate smoothly and the movement in the stroke direction became smooth.

Clean the inner surface where the plunger fits with a cotton swab while spraying parts cleaner from the negative pressure chamber side. If there is still a rough feeling here, it could become abrasive and damage the plunger again.

The by-starter is mounted on each carburetor. One thing to watch out for here is the quadruple carburetor for four cylinders. The plunger on each carburetor is actuated by a link or shaft, but the movement of the link or shaft may not be linked to the movement of the plunger.

The movement of the link is transmitted to the plunger via the lever, but there is a certain degree of freedom between the lever and the plunger, and structurally it is possible that the plunger is not fully closed even if it is fully closed on the link.

We have mentioned dust and dirt as causes of poor plunger movement, but there is also the effect of altered petrol on motorcycles that have been unridden for a long time. When liquid gasoline volatilizes, it adheres as a highly viscous varnish. When cleaning a dirty carburetor, we work very hard to remove the varnish from the float chamber and jets. The by-starter system is not as gasoline-soaked as the inside of the chamber, but it is possible that there is still some altered gasoline in the passages. In fact, it's not uncommon to try to operate the choke lever on a motorcycle that's been left for a long time and find that the plunger is stuck and won't move at all.

Removing the jets, needles, and negative pressure pistons can be done in a quad, but removing the plunger requires disassembling all the connected carburetors, which is a pain in the ass (some carb types allow the plunger to be removed in a quad). If you take the four carburetors apart, you'll have to replace the O-rings on the fuel lines that connect the carburetors together, and you'll have to re-sync the throttle valves. However, if the plunger is still stuck with altered gasoline, the possibility that the slow mixture will not be stable will not disappear.

If the exhaust gas smells like gasoline at idle, if the pilot screw and slow jet are correct but the slow system is slightly covered up, or if only a certain plug out of four cylinders is covered up, check the condition of the bi-starter function, especially the plunger. If the plunger is in good condition, check the condition of the plunger.